Implantable medical systems that are designed to deliver electrical stimulation, for example, to cardiac muscle or the spinal cord, typically include a pulse generator device to which one or more elongate electrical leads are coupled, for example, like the exemplary system schematically shown in FIG. 1A. FIG. 1A illustrates the system including a device 100, which is implanted subcutaneously in a pectoral region of a patient 12, and a pair of leads 110, which are connected to device 100. Leads 110 are shown extending distally from device 100, and into the venous system of patient 12, so that electrodes thereof are positioned for cardiac sensing and stimulation. A terminal connector assembly of each lead 110 is plugged into a corresponding receptacle of an insulative housing 101 of device 100, for electrical connection to device connector contacts, which are mounted within the receptacles. FIG. 1A further illustrates device 100 including a conductive enclosure 103 to which insulative housing 101 is attached. With reference to FIG. 1B, which is a plan view of device 100 having a portion of enclosure 103 removed, electronics 14 and an associated power source 16, for example, a battery assembly, are contained within conductive enclosure 103, and a feedthrough assembly 115 (FIG. 4), such as is known in the art, electrically couples electronics 14 to the aforementioned connector contacts.
With further reference to FIG. 1A, the comfort of patient 12 may be enhanced if a volume of device 100 is minimized. Yet the operational life of device 100 depends, at least in part, upon the longevity of power source 16, which, if power source 16 is a battery assembly, can directly depend upon the battery cell volume thereof—the larger the volume of the battery cell, the longer the life thereof. Thus there is a need for new implantable medical device constructions that increase packaging efficiency, so that the volume of the power source may be increased without significantly increasing an overall volume of the device.